1. Field of the Invention
This invention relates to removal of a scale from the surface of an article and, more particularly, to the removal of oxide from internal passages within an article.
2. Description of the Prior Art
As improved performance in modern gas turbine engines was achieved by increasing the temperatures experienced in the turbine section, it was found desirable by designers to change from solid components such as turbine blades to air cooled configurations. Generally included in the interior of such air cooled or hollow components is a serpentine cooling fluid path and a plurality of openings in interior walls as well as in exterior walls to provide for the passage and efflux of cooling fluid within and from the interior of such article. One typical fluid-cooled airfoil for use in high temperature conditions is shown in U.S. Pat. No. 3,628,885-Sidenstick et al patented Dec. 21, 1971 and assigned to the assignee of the present invention. The disclosure of such patent is incorporated herein by reference. As can be seen from the complexity of design, such a component is costly to manufacture.
Because such components as turbine airfoils are intended to operate under strenuous environmental conditions at elevated temperature, oxide, silicate and sulfide scales can collect on external surfaces as well as on internal passage surfaces. Such scales are particularly critical on internal surfaces because they can adversely affect the flow of cooling fluid, particularly through relatively narrow openings or passages in internal and external walls, and thus can adversely affect heat transfer characteristics of the component.
Repair processes for such costly fluid-cooled components as turbine blades and vanes have attempted to remove such scale from external surfaces through use of mechanical means such as grit blasting, abrasive rotating tools and ultrasonic vibration for external surfaces. Such mechanical methods are virtually ineffective on internal surfaces such as cooling passages. Acid leachants have been evaluated with the result that excessive attack on the metal of the component enlarged cooling passages, particularly fine openings, beyond acceptable limits and adversely affected the cooling characteristics of the component. In addition, fluoride vapor treatment for oxide removal can result in removal of some aluminum from the surface being treated.